1. Field of the Invention
The present invention relates generally to computer systems and, more particularly, to an apparatus and method for transmitting signals, such as Ethernet signals, which meet government emission standards, from a computer system to a transmission medium, such as a twisted pair cable.
2. Description of the Related Art
Many computer systems today are utilized in a networked configuration where each networked computer can transmit data to other computers on the same network. Various systems and related protocols have been developed over the years to implement such networks, such as Token Ring, Ethernet, and ATM. Depending upon which network is being used, certain requirements must be met, such as the types of hardware used and particular data characteristics.
The Ethernet local area network (LAN) is one of the most popular and widely used computer networks in the world. Since the beginnings of the Ethernet in the early 1970's, computer networking companies and engineering professionals have continually worked to improve Ethernet product versatility, reliability and transmission speeds. To ensure that new Ethernet products were compatible and reliable, the Institute of Electrical and Electronic Engineers (IEEE) formed a working group to define and promote industry LAN standards. Today, the IEEE has various Ethernet working groups that are responsible for standardizing the development of new Ethernet protocols and products under an internationally well known LAN standard called the "IEEE 802.3 standard."
Currently, there are a wide variety of standard compliant Ethernet products used for receiving, processing and transmitting data over Ethernet networks. By way of example, these networking products are typically integrated into networked computers, network interface cards (NICs), routers, switching hubs, bridges and repeaters. Until recently, common data transmission speeds over Ethernet networks were 10 megabits per second (Mbps). However, to meet the demand for faster data transmission speeds, in May 1995 the IEEE 802.3 standards committee officially introduced another standard, the "IEEE 802.3u standard," for a 100BASE-T system capable of performing data transmissions at up to about 100 Mbps. When operating with unshielded twisted pair (UTP) cable as a transmission medium, these networks are commonly referred to as 10BASE-T and 100BASE-T networks.
FIG. 1A is a diagrammatic representation of two computers 102, 104, which are connected through a network 105. The network 105 can include, for example, other computers, network hubs, network routers, servers or the like. Of course, a single cable connecting the computers 102 and 104 can alternatively be used. Each computer 102 and 104 includes systems to facilitate exchange of information to and from the computer. These systems are diagramatically illustrated by an open systems interconnection (OSI) layered model 106, that was developed by the International Organization for Standards (ISO) for describing the exchange of information between layers. The OSI layered model 106 is particularly useful for separating the technological functions of each layer, and thereby facilitating the modification or update of a given layer without detrimentally impacting the functions of neighboring layers.
Multiple layers (not shown) defined in the OSI model 106 are responsible for various functions, such as providing reliable transmission of data over a network; routing data between nodes in a network; initiating, maintaining and terminating a communication link between users connected to the nodes; performing data transfers within a particular level of service quality; controlling when users are able to transmit and receive data depending on whether the user is capable of full-duplex or half-duplex transmission; translating, converting, compressing and decompressing data being transmitted across a medium; and providing users with suitable interfaces for accessing and connecting to a network. Further, the lower portion of the OSI model 106 includes a media access control layer (MAC) 107 which generally schedules and controls the access of data to a physical layer (PHY) 108.
At a lowermost layer of OSI model 106, PHY layer 108 is responsible for encoding and decoding data into signals that are transmitted across a particular medium, such as a cable 110. To enable transmission to a particular medium, the PHY layer 108 includes a physical connector which is configured and operable to receive the cable 110. Also, the cable 110 can take various forms, including that of an unshielded twisted pair (UTP) cable.
When signals are passed through the cable 110 from the PHY layer 108, the potential exists for portions of the signal to emit from the cable 110 when it is an unshielded type, such as a UTP cable. More specifically, the portions which may emit from the cable typically are high frequency components of the signal. Because such emissions can interfere with other electrical devices in the vicinity of the cable 110, the U.S. government has developed stringent emission standards (commonly known as FCC Class A Requirements) to avoid such interference. To comply with such standards, in the PHY layer the high frequency signal components are typically removed from the primary signal before transmission on the cable 110. As is known in the art, this is commonly referred to as transmit pulse shaping that is followed by reconstruction filtering.
Ethernet transmitters have typically utilized a configuration such as that shown in FIG. 1B to remove high frequency components from the signal before transmission through cable 110. FIG. 1B schematically depicts one PHY application of an Ethernet device, specifically an Ethernet card 150. The Ethernet card 150 includes a PC board 152 on which a transmission system, formed by various components, is mounted. Included in these components is a packaged silicon chip 154, a filter 156, a transformer box 158, and a connector 160.
The packaged silicon chip 154 is configured to convert the input binary data from the host (e.g., a computer into which the Ethernet card 150 is mounted) to a signal that can be transmitted to the cable 110. This typically is accomplished by a data converter such as a Manchester encoder 162 and a digital-to-analog converter (DAC) 164 that is integrated on the packaged silicon chip 154. These devices alternatively can be located on separate semiconductor chips that are each mounted onto the PC board 152.
The Manchester encoder 162 outputs a signal having voltage swings that correspond to the binary data. The DAC 164 then converts the digital signal voltage from the Manchester encoder 162 to an analog signal voltage utilizing a reference voltage, Vref 165. Unfortunately, due to power supply or manufacturing process variations, the reference voltage level that is internally generated can vary by as much as about 20%, which can lead to inaccurate and inconsistent signals. In an Ethernet system, this would result in not matching an "Ethernet eye" template, which is a desired Ethernet transmission characteristic.
Electrically connected to the packaged silicon chip 154, the filter 156 operates to remove the high frequency components from the signal passed from the silicon chip 154. Typically, the filter 156 is formed from discrete components located on the PC board 152, such as inductor components 166 and capacitor components 168 used to form an inductor-capacitor (LC) low pass filter, as shown in FIG. 1B. Because discrete components typically require a substantial amount of PC board 152 area for proper layout and routing, the filter 156 tends to occupy a much larger area on the PC board 152 relative to the space utilized by the Manchester encoder 162 and DAC 164 of the packaged silicon chip 154.
The filtered signal passes from the filter 156 through a transformer 170 in transformer box 158, and then to the connector 160 which is configured to receive the cable 110. Although some typical Ethernet systems use a separate filter and a separate transformer, other systems may be in the form of a single module (not shown), which physically incorporates both the filter 156 and transformer 170.
Unfortunately, typical systems do not adequately control the impedance of the signal transmitted to the cable 110 with on-chip resistors which are subject to fabrication variations. Without adequate impedance control, the output voltage levels, specifically the peak-to-peak voltage level (Vpp), may vary beyond acceptable levels. By way of example, for transmission over differential pair lines, such as a UTP cable, such variation may be undesirable, resulting in poor transmission characteristics.
In addition to the components depicted in FIG. 1B, the PC board 152 typically includes several other components. For example, the PC board 152 may further include a processor, terminal circuitry, wiring, routing, connectors to the host (e.g., a computer system), and other semiconductor chips for performing the functions of other layers of the OSI model 106. Also, many of these components require their own routing and integration elements, which uses more space on the PC board 152. Further, as additional functions and components are developed, more space will be needed on the PC board to accommodate those components.
In addition, the marketplace is driving the development of increasingly smaller computers, requiring corresponding decreases in various computer components. This includes a desire to decrease the size of Ethernet systems, such as Ethernet cards. However, any decrease in size of such systems is limited by the relatively large area necessary for routing and integrating the discrete components of the filter 156.
In view of the foregoing, there is a need for methods and apparatuses for Ethernet signal transmission that will utilize less space than current systems. Further, it is desired to have a method and apparatus that better controls the impedance of the signal that is output to a transmission cable. In addition, it is desirable to have a method and apparatus that responds more robustly to power supply and manufacturing process variations that may cause the internally generated voltages to vary by up to about 20%.